The present invention generally relates to a contact detector for a machine tool and, more particularly, to a contact detector for detecting the contact between a motor-driven tool and a workpiece to be machined thereby for issuing an electric signal indicative of the start of machining performed by the machine tool on the workpiece.
In the determination of the start of contact between a workpiece to be machined and a motor-driven tool for the purpose of initiating the machining operation, it is well known to make use of an electromagnetic phenomenon created upon contact of the motor-driven tool with the workpiece to be machined, and so is a device for detecting the contact therebetween. By way of example, the Japanese Patent Publication No. 48-48861, published in 1973, discloses a system wherein a source of alternating current is connected between an upper end of a main shaft of the machine and a bed for supporting the workpiece and a detecting coil secured to the main shaft detects an alternating current flowing between the workpiece and the workpiece on the bed upon contact of the tool and the workpiece. According to the system disclosed in this publication, the electric current constantly flows through the machine framework at all times, and accordingly, this tends to constitute a major cause of the increased consumption of electric power and the increased noises.
According to the Japanese Patent Publication No. 58-41983, published in 1983, there is disclosed a detecting system for detecting the contact of the motor-driven tool with the workpiece by the utilization of changes in impedance of the coil, secured to the main shaft of the machine tool, which occur upon contact of the tool with the workpiece. The system disclosed in this second-mentioned publication is advantageous in that an unnecessary consumption of electric power can be minimized. However, it has been found that, since the magnitude of change in impedance of the coil is very small, the signal-to-noise (S/N) ratio is unsatisfactory and the machine operation tends to lack stability.
In addition to the respective problems inherent in these prior art detecting systems, the both have another problem common to them. Since the motor-driven tool used in the machine is driven at a high speed, an oil film is developed in bearings supporting the motor-driven tool, which oil film oftentimes act as an electric insulator insulating the motor-driven tool from the machine framework. Accordingly, in any one of these prior art detecting systems discussed above, the provision of, for example, contact brushes or the like is required to keep the motor-driven tool in electric contact with the machine framework during the high speed rotation of the motor-driven tool. This means that in these prior art detecting systems, the time-consuming and cost-incurring procedures are required to attach and maintain the constact brushes or the like.